Thyasira gouldi

Researched By	Angus Jackson	Data Supplied By	MarLIN
Refereed by	This information is not refereed.
Taxonomy
Scientific name	Thyasira gouldi	Common name	Northern hatchet shell
MCS Code	W1838	Recent Synonyms	None
Phylum	Mollusca	Subphylum
Superclass		Class	Pelecypoda
Subclass		Order	Veneroida
Suborder		Family	Thyasiridae
Genus	Thyasira	Species	gouldi
Subspecies
Additional Information	The larger eggs and characteristic sperm are useful features for separating Thyasira gouldi from Thyasira flexuosa.
Taxonomy References	Howson & Picton, 1997, Blacknell & Ansell, 1975, Tebble, 1966, Southward, 1986, Blacknell & Ansell, 1974
General Biology
Growth form	Bivalved	Feeding method	Symbiont contribution, Active suspension feeder
Mobility/Movement	Burrower	Environmental position	Infaunal
Typical food types	Suspended organic matter and digestion of mutualistic bacteria.	Habit	Burrow dwelling
Bioturbator		Flexibility	None (< 10 degrees)
Fragility	Intermediate	Size	Very small(<1cm)
Height	Insufficient information	Growth Rate	1 mm/year
Adult dispersal potential	Not researched	Dependency	Mutualist with/on/in
Sociability	Solitary
Toxic/Poisonous?	No
Additional Information	Thyasira gouldi has been found up to 1,500 individuals per square metre but typically below 500. Such abundances may no longer exist in Scottish waters. It is difficult to define an adult size range as there appears no specific point where juveniles become adult. Values provided are roughly maximum size where size refers to shell length. Large numbers of a mutualistic bacterium live sub-cuticularly in the gills of Thyasira gouldi (and several other thyasirids). The bacteria are chemoautotrophic and oxidise sulphur in order to assimilate carbon dioxide. Carbon isotope ratios indicate that digestion of these bacteria contributes considerably to the nutrition of this species. Although the bacteria utilise sulphur the bivalves inhabit sediment with very little free sulphide. The relationship is not thought to be obligate but the presence of the bacterium is very beneficial to the brachiopod. Eleven percent of a population of Thyasira gouldi in Loch Etive was infected with the parasitic copepod Axinophylus thyasirae Blacknell & Ansell, 1975). This parasite inhabits the mantle cavity and causes lower body weights and indirect castration. The female parasites reach sizes of 4.5 mm and there can be up to five parasites per host causing massive restriction of the cavity and interfering with feeding currents.
Biology References	Blacknell & Ansell, 1975, Southward, 1986, Blacknell & Ansell, 1974, Southward & Southward, 1991, Dando & Southward, 1986, Anonymous, 1999(g)
Distribution and Habitat
Distribution in Britain & Ireland	At the head of Loch Etive, west coast of Scotland. Formerly also found in Lochs Linnhe, Eil and Sunart. Recorded also from Shetland (further detail lacking). Presence in Scottish waters forms the extreme southern end of the geographic range.
Global distribution	A pan-arctic distribution from waters of the Commonwealth of Independent States along the north coast of Norway, around the coast of Greenland. On American coasts as far south as Cape Cod on the east and California on the west coast.
Biogeographic range	Not researched	Depth range	15-25m
Migratory	Non-migratory / Resident
Distribution Additional Information	Geographic distribution was probably more general during the last glaciation and remaining populations are relicts. The populations in Lochs Linnhe and Eil have been killed by the discharge of pulp-mill effluent. The population in Loch Etive has also decreased massively between 1984 and 1989. It is possible that this decrease has been brought about by a viral infection of the mutualistic bacteria living on the gills of Thyasira gouldi. Digestion of the bacteria provides considerable nutrient input. This species can burrow up to ten times its shell length (max. 8cm) and uses its vermiform foot to create channels deeper into the sediment. A mucus lined inhalant tube is made up to the surface from the living chamber. Little information is available about preferred water flow rates but are probably quite low being at the head of a sea loch. Wave exposure preferences are also likely to be sheltered. Typical depths in Scottish waters are 15-25 metres but the species has been found down to a few hundred metres depth. Optimal salinity levels are 25-30 psu. Thyasira gouldi appears to be restricted to locations where bottom waters remain cool throughout the year as a result of salinity stratification.
Substratum preferences	Muddy sand, Mud, Sandy mud	Physiographic preferences	Sealoch
Biological zone	Lower Infralittoral, Upper Circalittoral, Lower Circalittoral	Wave exposure	Insufficient information
Tidal stream strength/Water flow	Insufficient information	Salinity	Reduced (18-30 psu)
Habitat Additional Information
AMBI Group (Borja et al., 2000)	I
Distribution References	Blacknell & Ansell, 1975, Southward, 1986, Blacknell & Ansell, 1974, Bowden & Heppel, 1973, Southward & Southward, 1991, Dando & Southward, 1986
Reproduction/Life History
Reproductive type	Gonochoristic	Developmental mechanism	Direct Development
Reproductive Season	Year round	Reproductive Location	Sediment surface
Reproductive frequency	Annual protracted	Regeneration potential	No
Life span	Insufficient information	Age at reproductive maturity	Insufficient information
Generation time	Insufficient information	Fecundity	Up to 750 eggs per spawn
Egg/propagule size	Ca 260 µm long	Fertilization type	Internal
Larvae/Juveniles Larval/Juvenile dispersal potential <10m Larval settlement period Not relevant Duration of larval stage Not relevant
Additional Information	The sexes are separate and fertilization probably occurs in the mantle or suprabranchial cavity. Egg development is temperature dependent being (in the laboratory) around 50 days at 10 degrees C and 37 days at 16 degrees C. There is no synchronisation of reproduction and spawning occurs throughout the year. Eggs are white, oval and about 260 microns long. Up to 750 eggs are produced with each spawning. No information is available on the mechanism of spawning or the number of spawnings per year. Fertilised eggs are 'pumped' out of the inhalant tube and being quite dense, sink down onto and stick to the sediment about 1 cm from the opening. Consequently eggs are rarely dispersed by water currents. No information is available about life span but given the known growth rate and maximum size achieved it must be at least 5-10 years.
Reproduction References	Blacknell & Ansell, 1975, Blacknell & Ansell, 1974